8 Chapter masters and hundreds and hundreds of untrained recruits - How long were they in the warp? It does not mention, it could have been 1,000s of years.
You would either have to assume that he meant thousands and thousands of untrained recruits, and forgot to correct it, or they allowed for the fact that the serfs and population transferred to the Moon of Titan to assist in the process were actually breeding and their off spring who were deemed worthy enough for the testing, would then be selected as recruits. A third option is that they cloned the successful neophytes and created new recruits to start the process over again.
I do not think we are suppose to believe that they used some advance form of magic sorcery to create new recruits from thin air. Cloning can not be relied on or used too much as inherent mutations/gene coding errors are created over time if the process is continually used. This would then create more head-aches for those trying to keep the process running smoothly. So cloning could be a short term method used to (fill the order - 1000 Space Marines - CHECK, 1000 suits of armour - CHECK - - - ).